Method for regulating a roasting furnace

ABSTRACT

The invention relates to a method of regulating a roasting furnace in fluidized bed roasting. Part of the roasting furnace grate is separated off into a separate grate section, known as the overflow grate, where the nozzles and the amount of roasting gas blown through them can be regulated independently of the main grate. It is advantageous to position the separately regulated grate in the section of the furnace where the overflow aperture is located.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of regulating a roastingfurnace in fluidized bed roasting. Part of the grate of the roastingfurnace is separated into a separate grate section, known as an overflowgrate, where the nozzles and the amount of roasting gas blown throughthem can be regulated independently of the main grate. It isadvantageous to position the separately regulated grate in the sectionof the furnace where the overflow aperture is located.

2. Description of Related Art

The roasting of concentrates such as zinc sulphide concentrate usuallytakes place using the fluidized bed method. In the roasting process, thematerial to be roasted, a fine-grained concentrate, is fed into theroasting furnace via the feed units in the wall of the furnace above thefluidized bed. On the bottom of the furnace there is a grate, via whichoxygen-containing gas is fed in order to fluidize the concentrate. Thegrate usually has in the order of 100 gas nozzles/m². As the concentratebecomes fluidized, the height of the feed bed rises to about half thatof the fixed material bed. The height of the bed is on average 8-12% ofthe total height of the furnace. The pressure drop in the furnace isformed by the resistance of the grate and that of the bed. Theresistance of the bed is more of less the mass of the bed when the bedis in a fluidized state. The pressure drop is in the range of 240-280mbar.

The concentrate in the fluidized bed is oxidized (burnt) to a calcine bythe effect of the oxygen-containing gas fed via the grate, e.g. zincsulphide concentrate is roasted into zinc oxide. In zinc concentrateroasting the temperature to be used is in the region of 900-1050° C. Thecalcine is partially removed from the furnace through the overflowaperture, and partially it travels with the gases to the waste heatboiler and from there on to the cyclone and electrostatic precipitators,where the calcine is recovered. In general the overflow aperture islocated on the opposite side of the furnace to the feed units. Thecalcine removed from the furnace is cooled and ground finely forleaching.

For good roasting it is important to control the bed, i.e., the bedshould be good and the fluidizing controlled. Combustion should be ascomplete as possible and the calcine should come out of the furnacewell. The particle size of the calcine is known to be affected by thechemical composition and mineralogy of the concentrate as well as by thetemperature and oxygen enrichment of the roasting gas. Good fluidizingand bed stability can be improved, for example, by regulating the amountof impurities in the concentrate mixture or by adding water to the fineconcentrate, causing micropelletization. U.S. Pat. No. 5,803,949describes the stabilization of a fluidized bed in zinc concentrateroasting, where the bed is stabilized by regulating the particle sizedistribution of the bed.

The actual pressure drop of the roasting furnace is determined by theparticle size and the volume weight of the concentrate in the fluidizedbed, the height of the bed in the roasting furnace and the gratestructure. In order for the functioning of the roasting furnace to bestable, the pressure drop should remain in a certain position in thefurnace. A low pressure drop may be the result of a low bed for example.Thus local hot points may form and sintering may occur.

Conventionally furnace pressure drop and bed height are regulated byadding or removing baffle bars located at the lower edge of the overflowaperture. Pressure drop can also be affected somewhat by the amount ofgas fed through the grate, in particular the part caused by the grateitself. Adding and removing baffle bars may come to the limit and on theother hand, handling the bars themselves is not to be recommended forreasons of industrial hygiene.

BRIEF SUMMARY OF THE INVENTION

A method has now been developed according to the present inventionallowing roasting furnace conditions to be regulated, when material forroasting is fed above the fluidized bed and the fluidizing roasting gasthrough the grate at the bottom of the roasting furnace, and at leastsome of the calcined material is removed from the overflow aperturelocated at the height of the top of the fluidized bed. Part of thefurnace grate is separated off to form a separate section, known as theoverflow grate, where the nozzles and amount of gas blown through themare regulated independently of the main grate. The separately regulatedgrate is located in the same section of the furnace as the calcineoverflow aperture, preferably below the overflow aperture. The essentialfeatures of the invention will become apparent in the attached patentclaims.

It has been shown that using a separately regulated grate the ratio inwhich the calcine is removed from the furnace via the overflowaperture/boiler can be regulated. Using an overflow grate can affect theincrease of favourable particle size. It has been found that an overflowgrate can be used to regulate furnace conditions even if there were onlyless than 0.5% of all the nozzles in the grate in its area. The controlrange of the pressure drop of the overflow grate itself shouldpreferably be wide, around 200-2500 mbar.

In practice it has been noticed that increasing overflow grate pressuredrop increases the amount of calcine removed via the overflow aperturein relation to the amount of calcine recovered from elsewhere. On theother hand the capacity of the furnace can also be raised by routing alarger amount of the calcine via the overflow aperture and this can beachieved precisely by using the overflow grate. Increasing overflowgrate pressure drop may affect the turbulence of the fluidized bed,which causes the coarser material in the lower part of the bed to riseupwards and to be discharged from the furnace through the overflowaperture.

The calcine removed from the overflow aperture is cooled preferably in avortex cooler. It is known in the prior art that the sulphate content ofcalcine obtained from a boiler is higher than that recovered from avortex cooler. Calcine containing sulphates can cause blockages in theboiler, so decreasing the amount of calcine obtained from the boileraids the smooth functioning of the boiler and the whole process.

The invention is described by the following examples:

EXAMPLE 1

A production-scale roasting furnace was run with a constant amount ofair (42 000 Nm³) and standard baffle bars with a combined height of 75mm. The temperature was held constant at 950° C. and the feed mixturewas also kept constant. It was possible to regulate the furnace pressuredrop by regulating the pressure drop of the overflow grate as shown inthe table below:

TABLE 1 Overflow grate pressure drop Roasting furnace pressure drop mbarmbar  500 263 1 000 254 1 200 249

EXAMPLE 2

A roasting furnace as in example 1 was used. Oxygen (500 Nm³) was addedto the grate air (44 000 Nm³), whereupon the pressure drop of thefurnace began to rise, but it was stabilized by raising the pressuredrop of the overflow grate from 800 mbar to 1200 mbar.

What is claimed is:
 1. A method for regulating roasting furnace conditions in a roasting furnace having a fluidized bed in which a material for roasting is calcined, comprising feeding the material for roasting into the furnace at a location above the fluidized bed, feeding a fluidizing roasting gas through nozzles within a main grate located at the bottom of the furnace to fluidize the material for roasting, removing from the furnace at least some of the calcined material through an overflow aperture located at the top of the fluidized bed, separating off part of the roasting furnace main grate, within the same section of the furnace that the overflow aperture is located and below the overflow aperture, to form a separate section, an overflow grate, and feeding fluidizing roasting gas through nozzles within the overflow grate at a rate that is independent of the rate that fluidizing roasting gas is fed though the nozzles within the main grate.
 2. A method according to claim 1, wherein less than 0.5% of the total nozzles in the main grate are in the overflow grate.
 3. A method according to claim 1, wherein the pressure drop of the overflow grate is regulated within the range of 200-2500 mbar.
 4. A method according to claim 1, further comprising adjusting the pressure drop of the furnace by regulating the pressure drop of the overflow grate.
 5. A method according to claim 4, wherein the pressure drop of the overflow grate is increased to decrease the pressure drop of the roasting furnace.
 6. A method according to claim 4, wherein the pressure drop of the overflow grate is increased to stabilize the pressure drop of the roasting furnace.
 7. A method according to claim 1, further comprising adjusting the amount of calcine to be removed from the overflow aperture by regulating the pressure drop of the overflow grate.
 8. A method according to claim 7, wherein overflow grate pressure drop is increased to increase the amount of calcine removed via the overflow aperture in relation to the amount of calcine recovered from elsewhere.
 9. A method according to claim 1, further comprising adjusting the particle size of the fluidized bed by regulating the pressure drop of the overflow grate.
 10. A method according to claim 9 wherein overflow grate pressure is increased to increase turbulence of the fluidized bed so that coarser material in a lower part of the fluidized bed rises upwards and is discharged from the furnace through the overflow aperture.
 11. A method according to claim 1, wherein the material to be calcined is a concentrate.
 12. A method according to claim 11, wherein the material to be calcined is a zinc sulphide concentrate.
 13. A method according to claim 12, wherein the zinc sulphide concentrate is calcined into zinc oxide at a roasting temperature between 900° C. and 1050° C.
 14. A method according claim 1, wherein the material for roasting is calcined at a roasting temperature between 900° C. and 1050° C. 